Polyimides constitute a class of valuable polymers being characterized by thermal stability, inert character, usual insolubility in even strong solvents, good mechanical and electrical properties and high Tg, among others. Their precursors are usually polyamic acids, which may take the final imidized form either by thermal or by chemical treatment.
Polyimides have always found a large number of applications requiring at least some of the aforementioned characteristics in numerous industries, and recently their applications have started increasing dramatically in electronic devices, especially as dielectrics. With continuously escalating sophistication in such devices, the demands on the properties and the property control are becoming rather vexatious.
Especially for the electronics industry, improvements of polyimides are needed in forming tough, pin-hole free coatings, having low dielectric constant, low moisture absorption, low linear coefficient of thermal expansion, and good mechanical properties among others. It is not usually possible to maximize all properties, since many of them are antagonistic. Thus, only a compromised solution is usually achieved by at least partially sacrificing one or more of these properties in order to maximize a desired one.
An especially important property for electronics, and other applications as well, is low linear thermal expansion coefficient. This is because in electronic components, differences in the expansion coefficients of the components that make up the electronic device can generate stresses in the device which may lead to premature device failure. As electronic components become ever smaller, control of stress becomes an ever greater concern, such that the thermal expansions of the various components of a device should be matched as closely as possible. Since the stress can generally be related to the product of the difference in thermal expansion of the components and the moduli of those components, control of these factors is important in minimizing stress. Polymers generally have much higher thermal expansion coefficients than other components which make up electronics devices, e.g. silicon, silicon dioxide, copper, aluminum, etc., so that often the large mismatch between polymer and the other components of the device can lead to high stresses within the device. Attempts to reduce the stresses between polymers and the other materials have generally focused on reducing the thermal expansion coefficient mismatch between materials, although it is also possible to reduce stress by reducing modulus.
In polyimides, low thermal expansion coefficient has generally been achieved by the use of a stiff, rod like backbone. An example of this is the polyimide based on 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylene diamine (PPD). This polyimide, depending on processing conditions, can have a linear thermal expansion coefficient in the plane of the film in the range of 3-4 ppm which closely approximates that of silicon such that the stress between silicon and polyimide can be very low. Although this polymer has a low thermal expansion coefficient as well as other desirable properties, it does not have low dielectric constant. My U.S. patent application Ser. No. 07/720,680, filed on Jun. 25, 1991, abandoned describes fluorinated, low thermal expansion coefficient polyimides based on 9-aryl-9-(perfluoroalkyl)-xanthene-2,3,6,7-tetracarboxylic dianhydride or 9,9-bis(perfluoroalkyl)-xanthene-2,3,6,7-tetracarboxylic dianhydride and benzidine derivatives which exhibit a unique combination of low thermal expansion coefficient, low moisture absorption and low dielectric constant. These polyimides, however, because of their very stiff, rigid backbone have relatively low elongation. It is therefore very desirable to provide polyimides which substantially maintain the desirable properties of low dielectric constant and low thermal expansion coefficient while providing improved tensile elongation (higher than 10%).
U.S. Pat. No 5,051,520, (Trofimenko) discloses monomers and polyimides in general based on 9-aryl-9-(perfluoroalkyl)-xanthene-2,3,6,7-tetracarboxylic dianhydride and 9,9-bis(perfluoroalkyl)-xanthene-2,3,6,7-tetracarboxylic dianhydride
Japanese Patent Application Publication Kokai Hei 2-60933 (Masaki Ishisawa et al., Pub. Date: Mar. 1, 1990) discloses certain compositions of polyimides containing benzidine derivative and derivatives containing fluorochains.
Japanese Patent Application Publication Kokai Hei 3-72528 (Tetsu Matsuura et al., Pub. Date Mar. 27, 1991) discloses certain compositions of polyimides containing benzidine derivatives and as well as 3,3',4,4'-biphenyltetracarboxylic dianhydride or 2,2-bis(3,4-carboxyphenyl)hexafluoropropane.
International Publication No. WO 91/01340 (Harris, Pub. Date Feb. 7, 1991) discloses benzidine derivatives with miscellaneous dianhydrides.
None of the above references describes, suggests or implies the combination of materials and critical requirements of the present invention; namely a polyimide comprising the structure: ##STR1## wherein R.sup.1 is aryl or R.sup.2,
R.sup.2 is --CF.sub.3, PA1 R.sup.3 and R.sup.4 are selected from the group consisting of --C.sub.m F.sub.2m+1, --C.sub.p H.sub.2p+1, and --OC.sub.p H.sub.2p+1 PA1 m is an integer 0-4, PA1 p is an integer 0-2, and PA1 q is an integer greater than 10, PA1 R.sup.2 is --CF.sub.3, PA1 R.sup.3 and R.sup.4 are selected from the group consisting of --C.sub.m F.sub.2m+1, --C.sub.p H.sub.2p+1, and --OC.sub.p H.sub.2p+1 PA1 m is an integer 0-4, PA1 p is an integer 0-2, and PA1 q is an integer greater than 10, PA1 R.sup.3 and R.sup.4 are in 2, 2'-positions, respectively, in the benzidine ring, and are selected from the group consisting of --C.sub.m F.sub.2m+1 and --C.sub.p H.sub.2p+1, PA1 m is an integer 1-4, and PA1 p is an integer 1-2. PA1 (a) a substrate which comprises a conductor, semiconductor, or insulator; and PA1 (b) a dielectric film in contact with the substrate, the dielectric film comprising a polyimide derived from a fluoroxanthene derivative, a benzidine ring derivative, and an effective amount of 3,3',4,4'-biphenyltetracarboxylic dianhydride or 2,2-bis(3,4-carboxyphenyl)hexafluoropropane as defined above.
the polyimide also comprising an effective molar amount of ##STR2## or a combination thereof replacing in the polyimide structure an equivalent molar amount of ##STR3## to render the tensile elongation of the polyimide higher than 10%, without raising the value of the linear coefficient of thermal expansion of the polyimide to higher than 25, and the value of the dielectric constant to higher than 3.